Thursday, February 21, 2013

A Tour of Plasma Physics in Downtown Cambridge

Inside the Alacator C-Mod
Image: Mike Garrett
While I was in Boston for the AAAS conference, I got the chance to tour MIT's Plasma Science and Fusion Center. Though there were amazing science presentations going on at the convention center that I hated to miss, the chance to see the latest in plasma research was too good to pass up. 

A plasma is essentially a really energized gas, and it's often called the fourth stage of matter. The particles in a plasma have so much energy that electrons can't bind with electrons, which gives these naked nuclei a net charge. Positive protons aren't canceled out by the negative charge of electrons, which lets scientists use magnetic fields to manipulate plasma taking all kind of different shapes. The teams at MIT's lab are working on a number of different experiments looking at ways to understand and harness the power of plasma.


Behind this unassuming door, off a small side street in Cambridge Massachusetts, is the MIT Plasma Science and Fusion Center. There's little indication that inside scientists are using some of the most powerful magnetic fields in the world to manipulate the same stuff that the Sun is made of.

 The first stop was the Alcator C-Mod tokamak. A tokamak is a kind of nuclear reactor that physicists hope will someday power cities the of the future. Inside the big blue cylinder is a hollow, doughnut shaped cavity where powerful magnetic fields start out by compressing disperse plasma ions into a dense ring. The ring heats up, and a nuclear reaction starts to take place when ions get close to each other. Usually two ions repel because they have the same charge, just like how two north pole magnets with the same poles repel each other. But if they have enough energy, the ions can overcome that repulsion and knock right into each other. When that happens, the strong nuclear force takes over, the two ions bond, releasing a lot of energy. In theory, if done right, the energy released would be more than the energy needed to run the machine, and the excess could be used to generate electricity. This is the key to building a fusion power plant. 


Unfortunately scientists are not nearly there. Fusion research has been going on for years, and scientists have made great strides, but the net gain in energy still eludes them. The Alcator C-Mod won't be the machine to power the future, but it's helped lay a lot of the theoretical groundwork for bigger tokamaks that might. The Alcator C-Mod itself might be shutdown soon because of proposed budget cuts to fusion research. 

The churning plasma gets extremely hot, and it has to flow smoothly, otherwise it might melt the machine. Inside this rectangular case is a laser that shoots inside of the tokamak to check that the magnetic field inside is keeping the scorching plasma away from the walls of the chamber. 

This is command central for the Alacator C-Mod. The screens display important information about how the reactor is functioning. This is a recording from an earlier run, the tokomak was offline when we were there. From left to right, the screens show a visual image of the inside of the chamber, how hot all of the surfaces of the chambers are getting, a cross-section of the magnetic fields con fining the burning plasma within, a rundown of radio frequencies also used to help shape the plasma, and chart of other important information like power usage, magnetic field strength and the like. 

All of the rooms we walked through were surrounded by thick concrete, a reminder that when these machines are running, no one is actually in the same room. The thick radiation shielding makes the lab feel like a kind of bunker in places, but it's necessary for safety. As we headed downstairs to the lab's linear accelerator, we first passed by a container holding some of its more radiological components.

A linear collider is a kind of particle accelerator that shoots a thin beam of energized particles at a target at the end of a straight pipe. There are ones around the world for different purposes, but the one here is mostly used here to test instruments and materials needed for various fusion experiments. It's entirely built and run by the graduate students at MIT. The beam starts in the blue box at the end. Inside an energized emitter releases charged particles, atomic nuclei with one neutron and one proton known as "deuterons." Using electric charges, they're hustled into the beam pipe where they'll be accelerated up to almost the speed of light. 

As the ions shoot down the beam pipe, they pick up energy as electrodes switch quickly between positive and negative charge. Magnetic fields focus the beam to a width smaller than a human hair. 

The deuterons shoot out of the tube at the bottom of the photo and hit the black spot at the end of the copper pipe. The speeding particles fuse to whatever sample is in the middle of the target, and eject neutrons off to the left, which instruments collect and measure. This neutron harvest is the main way that scientists hope to extract usable energy from fusion tokamaks. If they put a block of the right kind of material in the path of the whizzing neutrons, they'll hit it and transfer their energy into it. The block will heat up, and if it gets it hot enough, scientists can use it to boil water, which will turn a turbine and generate electricity.

We moved on to a cavernous room where some more of the lab's of the biggest experiments are housed. This is the Levitated Dipole Experiment, designed to study the way plasmas behave, rather than trying to use it as a power source. The idea is to try to better understand "space weather."In the upper atmosphere charged ions get trapped inside Earth's magnetic field, resulting in belts of radiation and the aurora. 

In essence, the LDX as it's called, simulates the magnetic field of a planet inside it's chamber. Magnetic fields trap plasmas inside it just like Earth traps ions ejected from the sun. What makes this experiment so cool, is in order to study the plasma at the center, scientists have to use powerful magnets to levitate a half-ton coil of wire in the middle when its running. It has to float in the center, because if there were supports holding the coil to the side, they would disrupt  the magnetic fields and ruin the experiment. 

The view inside the LDX's sixteen-foot chamber. The hole at the bottom is where the round coil rests when idle. When the experiment is running, a winch first raises the coil up to the middle of the chamber then lets it go, suspending it between two powerful magnets. The coil hovers in the middle of the plasma cloud, while scientists watch the round vortexes that form around it. 

The last experiment we stopped at was the Versatile Toroidal Facility, another basic physics experiment. The idea is to understand what happens when two magnetic field lines pinch together while plasma is trapped inside. It might sound esoteric, but it happens all the time on the surface of the Sun. The sun is essentially one giant ball of plasma gas with a powerful and convoluted magnetic field. Every once in a while the field lines get twisted around and pinch together, causing a huge mass of charge particles to shoot away from the surface. This is where the charged particles that get trapped in our atmosphere come from. On Earth we know this as a solar flare, and if powerful enough, they can disrupt cell phone, TV and radio signals. Physicists are essentially creating their own tiny solar flares in the lab. 

This long tube sticking out is the tail end of one of the instruments used to observe the plasma within.

This wire cage is one of the instruments used to see inside the experiment. Magnetic fields swirl around the criss-crossed copper strands, inducing electrical currents in the wires that physicists use to track the bigger magnetic field within. Temperatures inside the VTF are much cooler than a tokamak, so the copper wires can be embedded in the plasma without melting. 

6 comments:

  1. Thank you for the wonderful photos and descriptions.

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  2. Wonderful! By reading and looking at the pictures, I felt young again, as I researched with ions and plasmas.
    Thank you! :)
    Regards,
    Mauro.

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  3. Agreed, thank you for the virtual tour. Too bad plasma physics has never been embraced by any administration.

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  4. Thanks for writing this piece! This is the very place that I did my PhD thesis work while a physics grad student at MIT in the 1990's.

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  5. It is my dream to learn about/work/study somewhere like this!

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  6. It should have more investment in aneutronic fusion, because it is clean, safe and dense, carbon-free and neutron-free. http://www.youtube.com/watch?v=VUrt186pWoA

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